Exposure to high oxygen tensions has been shown to lead to increased production of free radicals such as superoxide (02 anion). We have postulated that superoxide dismutase (SOD) is essential in protecting against the toxic effects of oxygen. Current work is directed toward isolating and characterizing an unusual mitochondrial superoxide dismutase from the rat, and in working out immunohistologic techniques for the localization of both the copper zinc and the mitochondrial superoxide dismutase in rat lung tissue. Quantitative electron microscopy is being used to study the structural changes that occur during exposures to sublethal doses of oxygen. The use of this type of quantitative pulmonary pathology permits a more accurate correlation of the effects of oxidant exposure on lung structure and function.